In situ nitrogen doping of co-evaporated copper-zinc-tin-sulfo-selenide by nitrogen plasma

ABSTRACT

A method and apparatus for manufacturing a nitrogen-doped CZTSSe layer for a solar cell is disclosed. A substrate is mounted in a vacuum chamber. A plurality of effusion cells are placed within the vacuum chamber in order to evaporate copper, zinc, tin, sulfur, and/or selenium to form elemental vapors in a region proximate the substrate. An RF-based nitrogen source delivers a nitrogen plasma in the region proximal to the substrate. The elemental vapors and the nitrogen plasma form a gas mixture in the region near the substrate, which then react at the substrate to form a CZTSSe absorber layer for a solar cell.

BACKGROUND

The present invention relates to a method and apparatus formanufacturing a solar cell and, in particular, to controlling nitrogendoping levels in an absorber layer of a Cu₂ZnSn(S,Se)₄ (CZTSSe) solarcell.

A solar cell includes multiple layers of material, with each layerhaving a specific function with respect to operation of the solar cell.For example, the absorber layer of the solar cell is the light sensitivelayer which captures light from the sun and creates electron-hole pairswhich, if collected, produce an electrical current. Among otherselection criteria, the absorber layer must have an appropriate dopingdensity in order to achieve an optimal p-n junction (i.e. the junctionwhich produces a built-in voltage in the device) when interfaced withthe buffer layer. The doping density in the absorber is one parameterwhich can impact the voltage produced by the solar cell. An emergingmaterial known as Cu₂ZnSn(S,Se)₄ (also known as CZTSSe) has been shownto be suitable for use as the absorber layer of a solar cell. Typicalpreparation methods for CZTSSe rely on intrinsic point defects in thematerial to produce the desirable doping density described above.However, the nature of intrinsic doping in CZTSSe is not well understoodand is commonly difficult to control. Nitrogen has the potential toimpact the doping density in CZTSSe. Therefore, it is of interest tofind methods for incorporating nitrogen uniformly into CZTSSe.

SUMMARY

According to one embodiment of the present invention, a method ofmanufacturing a solar cell includes: placing a substrate of the solarcell in a vacuum chamber; placing elements of copper, zinc, tin, and oneor more of sulfur and selenium in the vacuum chamber at a controlleddistance with respect to the substrate; evaporating the elements to formelemental vapors in a region proximate the substrate; introducing anitrogen plasma into the region to form a gas mixture of the elementalvapors and the nitrogen plasma in the region; and depositing the gasmixture at a surface of the substrate to form a CZTSSe absorber layerfor the solar cell.

According to another embodiment of the present invention, an apparatusfor manufacturing a CZTSSe solar cell includes: a chamber for mounting asolar cell substrate; a plurality of effusion cells within the chamberconfigured to evaporate copper, zinc, tin and one or more of sulfur andselenium to produce fluxes of elemental vapors in a region proximate thesubstrate; and a radio frequency (RF) source configured to introduce anitrogen plasma into the region, wherein the elemental vapors and thenitrogen plasma form a gas mixture proximal to the substrate such thatthe vapors react at the substrate to form CZTSSe.

According to yet another embodiment of the present invention, a methodof forming a CZTSSe absorber layer of a solar cell including: mounting asubstrate in a vacuum chamber; evaporating copper, zinc, tin and one ormore of sulfur and selenium to produce fluxes of elemental vapors in aregion proximate the substrate; introducing a nitrogen plasma into theregion to form a mixture with the elemental vapors near the substrate;and reacting the copper, zinc, tin and one or more of sulfur andselenium to form the CZTSSe absorber layer of the solar cell.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 shows an apparatus suitable for co-evaporating the individualelements onto a substrate of a solar cell in one embodiment of thepresent invention;

FIG. 2 shows a concentration profile of nitrogen in a CZTSSe absorberlayer manufactured using conventional nitrogen ion-implantation methods;

FIG. 3 shows a concentration profile of nitrogen in a CZTSSe absorberlayer manufactured using the nitrogen plasma integration methods of thepresent invention;

FIG. 4 shows an electrical response diagram for solar cells illustratingthe effects of nitrogen plasma co-deposition techniques disclosed hereinon the electrical properties of the CZTSSe absorber layer; and

FIG. 5 shows a flow chart illustrating a method for manufacturing aCZTSSe solar cell according to the present invention.

DETAILED DESCRIPTION

FIG. 1 shows an apparatus 100 suitable for depositing elements onto asubstrate 110 of a solar cell in one embodiment of the presentinvention. The apparatus 100 includes a chamber 102 that can beevacuated so as to achieve high-vacuum (HV) or ultra-high vacuum (UHV)conditions, which are well-suited for thin film deposition. The chamber102 includes one or more effusion cells 104 a-e placed at controlleddistances and angles with respect to a substrate 110. An effusion cellis an HV or UHV device containing an appropriate crucible filled withthe desired material and a mechanism for heating the crucible andthereby evaporating the desired material. This vapor can then bedeposited on the substrate 110. In an illustrative embodiment, theeffusion cells 104 a-e include a copper (Cu) effusion cell 104 a (i.e.,an effusion cell that contains and evaporates copper), a zinc (Zn)effusion cell 104 b, a tin (Sn) effusion cell 104 c, and a sulfur (S)effusion cell 104 d. In some embodiments, a selenium (Se) effusion cell104 e may be used along with effusion cells 104 a-d. The sulfur effusioncell may include an additional feature known as a “cracking zone” whichthermally dissociates larger compounds such as S8, S4, etc. intoelemental sulfur. The selenium effusion cell may similarly include acracking zone. The sulfur and/or selenium cells may also be furtherequipped with a valve mechanism for controlling a flux of the elementalvapors of sulfur and/or selenium. Each effusion cell (104 a-e) heats thecrucible inside of it, which has been filled with the desired evaporantor material, to at or above the evaporation temperature of the materialinside the crucible. As a selected effusion cell (e.g., Cu effusion cell104 a) is heated at or above the evaporation temperature of the materialinside of it (e.g., Cu) a flux of elemental vapor is produced, whichflux is directed towards region 108 and the substrate 110. In region108, the elemental vapor (e.g., elemental Cu vapor) may mix with theother elemental vapors (e.g., elemental vapors of Zn, Sn, S, and/or Se)from the other effusions cells prior to deposition at the substrate 110.The evaporation rates of the elements may be controlled individually byvarying the temperature in each effusion cell in order to obtain a finalcompound with a selected composition. The chamber 102 further includes asource 124 for introducing a nitrogen plasma (N-plasma) 112 into theregion 108. The nitrogen plasma 112 introduced into region 108 mixeswith the elemental vapors of Cu, Zn, Sn, S, and/or Se in region 106. Thegas mixture 106 is subsequently deposited on the substrate 110 to form aCZTSSe absorber layer 125 (i.e., a light-sensitive layer) of the solarcell. While this embodiment is employs effusion cells, it is noted thata vacuum-compatible thin-film deposition source may be used.

In an alternate embodiment, the element (i.e., copper, zinc, tin andsulfur and/or selenium) may be introduced into the region usingsputtering techniques. The elemental vapors may be produced bybombarding a solid elemental source with impinging species, such asargon atoms, etc. Various parameters of the sputtering process may becontrolled to provide a selected atomic concentration in the region 108for each of the elements. In another embodiment, the source elements maybe “binary” materials that include combinations of the elements of theeventual absorber layer. Such binary materials may include, for example,Cu₂S, SnS, SnS₂, ZnS, etc. Such binary materials may be evaporated fromthe effusion cells described with respect to FIG. 1, or they may bereleased from sputtering targets.

Returning to FIG. 1, the nitrogen source 124 includes a nitrogen (N₂)reservoir 120 and a discharge tube 114 fitted with one or more apertureplates 116. The nitrogen source 124 includes a pathway for the nitrogengas from the reservoir 120 to reach the discharge tube 114. Aradio-frequency (RF) power coil 122 imparts energy (i.e., electricalpower) to the nitrogen gas at a level sufficient to break some of thebonds in the N₂ gas, thereby producing a reactive nitrogen plasma 112.In one embodiment, the RF coil 122 is fitted around the discharge tube114 through which nitrogen gas from the reservoir 120 passes into thevacuum chamber. The nitrogen plasma 112 then flows through the apertureplate(s) 116 into the region 108 to mix with the elemental gases inregion 108. The gas mixture 106 (including the elemental vapors of Cu,Zn, Sn, S and/or Se and the nitrogen plasma 112) is directed towards thesubstrate 110 and is deposited on the substrate 110 to form the absorberlayer 125 on the substrate 110. In HV or UHV thin film deposition withtypical configuration of sources, substrates, and angles there-between,the evaporated material is not expected to undergo significant gas-gascollisions prior to reaching the substrate due to the low pressureinside the chamber. Thus, reaction to form CZTSSe occurs mainly at thesubstrate 110. The distribution of evaporated material reaching thesubstrate 110 is governed by the source-to-substrate distance and theangle between source and substrate 110.

Various methods are used to control the flux of reactive nitrogen 112reaching region 108, and thus control the concentration of nitrogenexpected to incorporate into the absorber layer 125. The size anddensity of holes in the aperture plate 116 can be selected to controlthe flux of the nitrogen plasma 112 arriving in region 108. In oneembodiment, the cross-section of openings in the aperture plate 116 areselected to allow a controlled quantity of nitrogen (i.e., about 0.5atomic percent) to incorporate into the resulting absorber layer 125.The atomic density of nitrogen in the region 108 may further becontrolled by adjusting a flow rate of the nitrogen gas through thedischarge tube 114. The chemical characteristics of the nitrogen plasma(i.e. the extent to which all N₂ bonds have been broken) may becontrolled by adjusting the power of the RF coil 122.

FIG. 1 further includes a controller 130 configured to adjust thevarious operations of the apparatus 100, such as the evaporation rate ofone or more of the effusion cells 104 a-e, the flow rate of the nitrogengas, the RF power applied to the coil around the nitrogen gas, etc. Thecontroller 130 includes a processor 132 and a memory 134. The processor132 has access to the memory and to various programs 136 containedtherein. The processor 132 may perform the programs 136 in order tocontrol an operation of the apparatus 100. In various embodiments, thememory 134 may include a non-transitory computer-readable medium, suchas a solid state memory device.

FIG. 2 shows a concentration profile (200) of nitrogen in a CZTSabsorber layer manufactured using a conventional nitrogenion-implantation method. As shown in FIG. 2, the concentration ofnitrogen at the surface (202) of the layer is about 1.6×10²⁰ atoms percubic centimeter (atoms/cm³) and increases to about 5.4×10²⁰ atoms/cm³at a penetration depth of about 100 nanometers (nm) (204). However, forpenetration depths greater than 100 nm (204), the nitrogen concentrationdecreases with depth, so that the nitrogen concentration falls below1×10¹⁹ atoms/cm³ by a depth of 300 nm and is negligible at a depth ofabout 750 nm (206). This highly non-uniform distribution of nitrogenthroughout the CZTSSe layer may not be optimal for solar cell operation.

FIG. 3 shows a concentration profile (300) of nitrogen in a CZTSabsorber layer 125 manufactured using the nitrogen plasma incorporationmethods of the present invention. The absorber layer 125 is dopedin-situ with nitrogen by selecting an aperture plate (116, see FIG. 1)containing three holes through which the reactive nitrogen gas flows ata rate of 2 standard cubic centimeters per minute. The diameter of theholes may be between about 0.2 millimeters to about 1 millimeter. The RFpower used to generate the reactive nitrogen gas is supplied by an RFpower supply capable of delivering up to 600 Watts. Typical operationpower is between about 400 Watts and about 500 Watts. The resultingnitrogen concentration is substantially more uniform (2.16×10¹⁹atoms/cm³ or about 0.1 atomic percent) between the surface (penetrationdepth=0 nm) and a penetration depth of 800 nm, i.e., over the entirethickness of the absorber layer 125.

FIG. 4 shows an electrical response diagram for solar cells illustratingthe effects of nitrogen plasma co-deposition techniques disclosed hereinon the electrical properties of the CZTS-based solar cell. Curve 402represents a response of a CZTS-based solar cell manufactured withoutusing the N-plasma doping described herein. Curve 404 represents theresponse of a CZTS-based solar cell in which the CZTS film was preparedusing the N-plasma doping method of the present invention. Inparticular, the CZTS solar cell related to curve 404 is manufactured byCZTS co-evaporation with in-situ Nitrogen on a Molybdenum/Soda LimeGlass (Mo/SLG) substrate. Post-deposition annealing is then performed atgreater than 550° Celsius.

As shown in FIG. 4, the short-circuit current density of the devicedeposited in curve 402 (i.e., the current density of curve 402 when nobias voltage is applied) is about 15.5 mA/cm², as indicated at point412. The short-circuit current density of the device depicted in curve404 is about 17.2 mA/cm², as indicated at point 414. Thus, the CZTSlayer of the present invention provides an increased photocurrent overthe undoped CZTS layer. Additionally, the open-circuit voltage of thedevice depicted in curve 402 is about 383 mV (422) for the devicecontaining the undoped CZTS layer, whereas this value is about 631 mV(424) for the device containing the doped CZTS layer (curve 404) of thepresent invention.

Table 1 compares various properties of devices containing the doped CZTSlayer of the present invention compared to the devices containing anundoped CZTS layer. The row starting with “CZTS” displays theperformance characteristics of a device that employs a doped CZTSabsorber layer, and the row starting with “CZTS:N” displays theperformance characteristics of a device employing an undoped CZTSabsorber layer. The properties include efficiency (η, %), open-circuitvoltage (Voc, mV), short-circuit current density (Jsc, mA/cm²), fillfactor (FF, %), and series resistance (Rs, Ohm-cm²):

TABLE 1 Jsc H Voc (mV) (mA/cm²) FF Rs (Ohm-cm²) CZTS 5.6% 631 17.2 51.5%17.3 CZTS (N) 2.54% 383 15.5 42.8% 8.8(V_(oc) and J_(sc) are shown in FIG. 4 as well as in Table 1.) Theproperties shown in Table 1 show the performance advantages of devicesemploying the doped CZTS layer in a solar cell formed using the methodsdisclosed herein compared to those with an undoped CZTS layer formed viaconventional methods.

FIG. 5 shows a flow chart 500 illustrating a method for manufacturing aCZTS solar cell according to the present invention. In Block 502, asubstrate of the solar cell is placed in a chamber at a first location.In Block 504, effusion cells are placed at a controlled distance withrespect to the substrate. The effusion cells each contain one of:elemental Cu, Zn, Sn S, and/or Se. In Block 506, the effusion cells heatthe crucibles containing the target materials such that the materials(Cu, Zn, Sn, S, and/or Se) evaporate, thus releasing a flux of materialinto region 108 proximate the substrate 110. The temperatures in theeffusion cells are selected so as to achieve appropriate elementalfluxes (Cu, Zn, Sn, S, and/or Se) to obtain a desirable CZTSSe filmcomposition. In Block 508, a nitrogen plasma is introduced into thechamber at an angle such that it will mix with the other elementalvapors. In Block 510, the elemental vapors all impinge upon thesubstrate, thereby depositing and forming the CZTS absorber layer of thesolar cell.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of onemore other features, integers, steps, operations, element components,and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated

The flow diagrams depicted herein are just one example. There may bemany variations to this diagram or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order or steps maybe added, deleted or modified. All of these variations are considered apart of the claimed invention.

While the preferred embodiment to the invention had been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

What is claimed is:
 1. An apparatus for manufacturing a CZTSSe layer fora solar cell, comprising: a chamber for mounting a solar cell substrate;a copper effusion cell, a zinc effusion cell, a tin effusion cell and aone or more of sulfur and selenium effusion cells of a plurality ofeffusion cells within the chamber configured to evaporate copper, zinc,tin and one or more of sulfur and selenium to produce fluxes ofelemental vapors in a region proximate the substrate; a radio frequency(RF) source separate from the plurality of effusion cells, the RF sourceconfigured to introduce a nitrogen plasma into the region, wherein theelemental vapors and the nitrogen plasma form a gas mixture in theregion such that the vapors react at the substrate to form CZTSSe; and aprocessor configured to control the plurality of effusion cells toevaporate the copper, zinc, tin and one or more of sulfur and seleniumto obtain elemental vapors of a selected composition and to control theintroduction of the nitrogen plasma into the region.
 2. The apparatus ofclaim 1, wherein the plurality of effusion cells include binarymaterials composed of at least two of the elements of copper, zinc, tin,sulfur and selenium.
 3. The apparatus of claim 1, further comprising acontroller configured to control the evaporation rates of the elementsto obtain a selected CZTSSe film composition.
 4. The apparatus of claim1, wherein the RF source is configured to pass a nitrogen (N₂) gasthrough an RF coil to break bonds in the N₂ gas thereby to produce anitrogen plasma or reactive nitrogen gas.
 5. The apparatus of claim 4,wherein the RF source is fitted with at least one aperture plate, suchthat the nitrogen plasma passes through the aperture plate prior toreaching the reaction region.
 6. The apparatus of claim 5, wherein theRF source is further configured to control the flux and chemicalcharacteristic of the nitrogen plasma by controlling at least one of: anRF power; a flow rate of N₂; and a size and density of holes in theaperture plate.
 7. The apparatus of claim 6, wherein a diameter of theholes in the aperture plate is about 0.2-1.0 millimeters.
 8. Anapparatus for manufacturing a solar cell, comprising: a chamber formounting a solar cell substrate; a plurality of effusion cellscomprising a copper effusion cell, a zinc effusion cell, a tin effusioncell and a one or more of sulfur and selenium effusion cells within thechamber at a controlled distance with respect to the solar cellsubstrate, the plurality of effusion cells configured to evaporatecopper, zinc, tin and one or more of sulfur and selenium to producefluxes of elemental vapors in a region proximate the substrate; a radiofrequency (RF) source separate from the plurality of effusion cells, theRF source configured to introduce a nitrogen plasma into the region,wherein the elemental vapors and the nitrogen plasma form a gas mixturein the region such that the vapors react at the substrate to form aCZTSSe layer of the solar cell; and a processor configured to controlthe plurality of effusion cells to evaporate the copper, zinc, tin andone or more of sulfur and selenium to obtain elemental vapors of aselected composition and to control the introduction of the nitrogenplasma into the region.
 9. The apparatus of claim 8, wherein theplurality of effusion cells includes binary materials composed of atleast two of the elements of copper, zinc, tin, sulfur and selenium. 10.The apparatus of claim 8, further comprising a controller configured tocontrol the evaporation rates of the elements to obtain a selectedCZTSSe film composition.
 11. The apparatus of claim 8, wherein the RFsource is configured to pass a nitrogen (N₂) gas through an RF coil tobreak bonds in the N₂ gas thereby to produce a nitrogen plasma orreactive nitrogen gas.
 12. The apparatus of claim 11, wherein the RFsource is fitted with at least one aperture plate, such that thenitrogen plasma passes through the aperture plate prior to reaching thereaction region.
 13. The apparatus of claim 12, wherein the RF source isfurther configured to control the flux and chemical characteristic ofthe nitrogen plasma by controlling at least one of: an RF power; a flowrate of N₂; and a size and density of holes in the aperture plate. 14.The apparatus of claim 13, wherein a diameter of the holes in theaperture plate is about 0.2-1.0 millimeters.
 15. An apparatus formanufacturing a CZTSSe layer for a solar cell, comprising: a chamber formounting a solar cell substrate; a copper effusion cell, a zinc effusioncell, a tin effusion cell and a one or more of sulfur and seleniumeffusion cells of a plurality of effusion cells within the chamberconfigured to evaporate copper, zinc, tin and one or more of sulfur andselenium to produce fluxes of elemental vapors in a region proximate thesubstrate; a radio frequency (RF) source separate from the plurality ofeffusion cells, the RF source configured to introduce a nitrogen plasmainto the region, wherein the elemental vapors and the nitrogen plasmaform a gas mixture in the region such that the vapors react at thesubstrate to form CZTSSe; a controller configured to control theevaporation rates of the elements to obtain a selected CZTSSe filmcomposition; and a processor configured to control the plurality ofeffusion cells to evaporate the copper, zinc, tin and one or more ofsulfur and selenium to obtain elemental vapors of a selected compositionand to control the introduction of the nitrogen plasma into the region.16. The apparatus of claim 15, wherein the plurality of effusion cellsinclude binary materials composed of at least two of the elements ofcopper, zinc, tin, sulfur and selenium.
 17. The apparatus of claim 15,wherein the RF source is configured to pass a nitrogen (N₂) gas throughan RF coil to break bonds in the N₂ gas thereby to produce a nitrogenplasma or reactive nitrogen gas.
 18. The apparatus of claim 17, whereinthe RF source is fitted with at least one aperture plate, such that thenitrogen plasma passes through the aperture plate prior to reaching thereaction region.
 19. The apparatus of claim 18, wherein the RF source isfurther configured to control the flux and chemical characteristic ofthe nitrogen plasma by controlling at least one of: an RF power; a flowrate of N₂; and a size and density of holes in the aperture plate. 20.The apparatus of claim 19, wherein a diameter of the holes in theaperture plate is about 0.2-1.0 millimeters.